The present invention relates to an operation control method for a friction engagement element, such as a clutch, brake, etc., and more particularly, to operation control for quickly moving a piston of a frictional engagement element within its ineffective stroke sections.
Frictional engagement elements, such as clutches, brakes, etc., used in automatic vehicular transmission systems and the like are each composed of frictional engaging members, which are connected individually to input and output members, and engaging each other, and a hydraulically-operated piston urging the engaging members to engage each other. In supplying an oil pressure to the respective pistons of the frictional engagement elements so that the pistons are pressed against the engaging members to cause the engaging members on the input- and output-member sides to engage each other, it requires a long time (hereinafter referred to as "piston ineffective stroke time") for the pistons to travel through their dead or ineffective stroke from a full oil-pressure release position to a position just short of the engagement start position for the engaging members. In the meantime, the frictional engagement elements are subject to no engaging force. Due to delay of the start of the engagement of the clutches and the like, the transmission control operation of the automatic transmission systems requires a lot of time, thus entailing various problems on hydraulic circuit control.
In order to solve these problems, the maximum effective oil pressure (maximum oil pressure allowed to be practically supplied from a hydraulic control device to the pistons) should only be supplied to the pistons to shorten the stroke speed, for example. However, since the time required for each piston to move through its ineffective stroke from the oil pressure release position to the position just short of the engagement start position varies, depending on the magnitude of the oil pressure supplied to the piston, the pressurized oil temperature, the amount of wear of sliding portions of the piston after prolonged use, etc., so the supply time for the maximum effective oil pressure cannot be kept constant.